Axially assembled enclosure for electrical fluid heater having a peripheral compression ring producing a diametrically balanced force

ABSTRACT

A metal honeycomb heating element is protectively mounted in an enclosure comprising a one-piece enclosure wall, the heater being supported between a circumferential support member and a circumferential compression ring attached to the enclosure wall. A diametrically balanced compressive force is maintained on the edges of the heating element by the compression ring and support, this force operating to significantly extend the life of the heater element under hot vibration. Heater slot separators and electrode feedthrough structures for extending the life of the heater element are also provided.

BACKGROUND OF THE INVENTION

The present invention relates to electrically heated catalytic convertermodules for automotive exhaust gas treatment systems. More particularly,the invention relates to an improved design and assembly method for anelectric heater enclosure for such treatment.

A number of exhaust system designs are being considered to meet targetedexhaust emissions requirements for ultra-low emission vehicles (ULEVrequirements). Among the designs being considered are those employing anelectrically powered fluid heater positioned upstream and adjacent to anoxidation catalyst unit. The electric heater heats the exhaust gas andcatalyst unit during initial or "cold-start" engine operation, therebyadvancing the onset of catalytic oxidation and reducing the emission ofhydrocarbons (unburned fuel) otherwise emitted during that operation.

Attempts to contain and protect these heaters in the exhaust stream haveinvolved approaches similar to those used to mount or "can" ceramichoneycomb catalyst substrates. These include the use of axiallyassembled enclosures such as described in U.S. Pat. No. 4,207,661 toMase et al. Included in those enclosures are front and rear supportingmembers composed of a resilient material for supporting the catalyticconverter substrate within the enclosure while shielding it frommechanical shocks.

U.S. Pat. Nos. 4,142,864 and 4,413,392 disclose the packaging of similarceramic substrates by "stuffing" the substrates into cylindrical cansegments, followed by the attachment of retaining rings and or conicalend caps to the can segments. Axial assembly in the manner of the abovepatents is advantageous in that the number of components required tosecurely encase the catalytic substrate within the shielding metalcontainer is relatively small, and in that tight axial constraint of thesubstrates within the enclosures is easy to achieve.

However, substantial difficulties relating to service life have beenencountered in adapting the techniques of ceramic substrate canning tothe canning of extruded metal honeycomb heaters used for the preheatingof exhaust gases for subsequent catalyst treatment. This is largelybecause metal heater units, being composed of thin metal, are relativelyductile and thus somewhat more prone to vibration damage than ceramichoneycombs.

The durability requirements for metal heaters will be as stringent asthose for ceramic honeycombs used as automotive catalyst supports. Theenclosure system used must provide environmental and physical protectionadequate to enable the heaters to meet government mandated standards formaximum allowed levels of non-methane hydrocarbons, CO, and nitrogenoxides for up to 100,000 miles of automobile use. Up to 50,000 enginestarting cycles as well as severe thermal cycling, extreme temperatures,and high temperature vibration will be encountered by the heaters duringthis interval.

One previous approach to the containment of electrical honeycombheaters, described in a co-pending, commonly assigned patent applicationfor an "Axially Assembled Enclosure For Electrical Fluid Heater andMethod", filed Aug. 30, 1994 by L. S. Rajnik et al., uses a two-pieceenclosure wherein the heater is supported by compression between twohalves of a steel enclosure welded together. These can halves aredesigned to firmly support and protect the honeycomb and associatedinsulating matting material from vibration damage in use.

While approaches such as above have been effective to substantiallyextend the service life of these heaters, they have not fully met themost stringent service life requirements, particularly under the moresevere vibrational conditions of the hot exhaust environment.

It is therefore a principal object of the present invention to providean improved metal honeycomb heater assembly useful in the treatment ofengine exhaust gases which offers significant improvements in heaterdurability under conditions approximating the environment of use.

It is a further object of the invention to provide a canning methodapplicable to the canning of extruded metal honeycombs which provides adurably contained heater unit exhibiting high resistance to hotvibration damage over a prolonged period.

Other objects and advantages of the invention will become apparent fromthe following description.

SUMMARY OF THE INVENTION

The present invention arose from a study of the causes of vibrationalheater failure in heater enclosures employing edge compression forheater protection and support. In the case of two-piece enclosures suchas described above, wherein heater support consisted primarily of edgecompression axially applied to the heaters, our studies indicate thatfailure in vibration is due in many cases to insufficient edgecompression in the final assemblies, perhaps caused by the adverseeffects of final welding steps on stress distribution within theenclosures.

To overcome these difficulties the invention employs a one-pieceenclosure in combination with a novel axial mounting technique to securea uniform, predictable, and reproducible level of compressive force onthe heaters during assembly. The result is a significant and unexpectedimprovement in heater durability under both hot and cold vibration.

Still further improvements in heater durability are achieved through theuse of enhanced heater slot separation methods and elements, and betterheater electrode sealing configurations.

In a first aspect the invention comprises an electric heater module forheating a gas stream which includes a one-piece cylindrical metalenclosure formed by a wall member of closed-curved configuration. Withinthe channel formed by that enclosure is provided a circumferentialsupport member, connected with the inner surface of the wall andextending into the channel to form a first peripheral support surfacefor a heater.

A conductive metal honeycomb heating element is positioned within anddisposed across the channel, closely adjacent to the support member.That element is provided with at least one layer of refractory resilientinsulation material, disposed around its outer edge surfaces toprotectively separate and insulate it from wall and support surfaces.With this layer of insulation intervening, the peripheral(circumferential) edge of the heating element rests on and is supportedwithin the channel by the peripheral support surface of the supportmember.

In the channel next to the heating element and in proximity to itssecond or opposing edge surface is an annular compression ring. Like thesupport member adjacent the opposing peripheral surface of the heatingelement, this ring is attached to the inner surface of the wall andextends into the channel to form a second peripheral support surface forthe opposing heater edge surface, the later again being covered by oneor more layers of the insulation material.

To support the heating element in a manner insuring adequate protectionfrom motion damage, the compression ring is fastened to the inner wallof the enclosure at a location sufficiently close to the opposing heatersupport member to develop a predetermined level of pressure through theresilient mat material on the heating element. For the mountingcomponents and procedures of the invention that force level will be inthe range of about 620-2760 kPa (90-400 psi).

The invention additionally includes a method for mounting a metalhoneycomb heating element in a one-piece enclosure such as hereinabovedescribed. That method includes first providing a cylindrical metalenclosure in the form of a one-piece closed-curved wall member, the wallincorporating a circumferential support member on its inner surface. Thesupport member extends into the channel formed through the enclosure toform a first peripheral support surface for the heating element.

A conductive metal honeycomb heating element is positioned across thechannel adjacent to the support member but with at least one layer ofrefractory resilient insulation material being provided between theheating element and each of the wall member and first peripheral supportsurface. An annular compression ring is then placed in the channel inproximity to the heating element, again while providing at least onelayer of refractory resilient insulation material between the heatingelement and the compression ring.

To mount the heater in the enclosure the compression ring is urgedagainst the insulation material, heating element, and support member byapplying a balanced force to the ring at multiple discrete pressurepoints about the ring circumference. This balanced force is sufficientto develop a compressive force in the range of about 620-2760 kPa(90-400 psi) on the insulation material and heating element. Whilemaintaining the compressive force thus generated the ring is permanentlyattached to the enclosure wall at multiple attachment points about theperiphery of the heater. Point attachment, rather than seam welding ofthe ring to the enclosure wall, is used.

In a particularly preferred embodiment of the above method, the forceapplied to the compression ring during attachment is applied by a rigidmounting plate which has multiple radially-outwardly-extendingprotrusions contacting the compression ring at multiple discretepressure points about the ring circumference. The recesses between theseprotrusions expose the compression ring while the ring, insulation andheater element remain under compression, Accordingly, easy access to thejoint between the ring and enclosure wall, for convenient pointattachment of the ring to the wall while maintaining pressure on theheating element, is provided.

Electrical heater enclosures produced as above described can be designedwith features which permit convenient mating with other components ofthe exhaust system. For example, provision can be made for convenientmating with a light-off catalyst module, in order to provide a ruggedelectrically heated catalyst module as a single unit.

A particular advantage of the invention is that it significantly extendsthe service life of the heater element, due to the improved uniformityand degree of control over the pressure applied to the honeycomb heaterby the support member and compression ring. Maintaining uniform andcontrolled pressure in the final assembly minimizes the adverse effectsof thermal and mechanical stresses on heater integrity which have in thepast limited heater service life.

DESCRIPTION OF THE DRAWINGS

The invention may be further understood by reference to the appendeddrawings wherein:

FIG. 1 is a schematic end view in cross-section of an assemblycomprising an electrical honeycomb heater supported within a one-pieceenclosure in accordance with the invention;

FIG. 2 is a schematic side view in cross-section of the assembly of FIG.1;

FIG. 3 is a schematic side elevational view in cross-section of theassembly of FIGS. 1 and 2 during assembly with tooling for the mountingpre-compression of the heater within the enclosure;

FIG. 4 is a schematic top plan view of the assembly and tooling shown inFIG. 3;

FIG. 5 is a schematic top plan view of alternative tooling for use withthe assembly shown in FIG. 3;

FIG. 6 is a partial schematic cross-sectional view of heater slotseparation and electrode sealing means provided according to theinvention; and

FIG. 6a is an enlarged schematic illustration of a slot separator ofFIG. 6.

DETAILED DESCRIPTION

Electrical heating elements useful in the practice of the inventioninclude any of the heater types being developed for the electricalheating of exhaust gas effluents. The preferred heaters are extrudedmetal honeycombs, examples of which are disclosed in U.S. Pat. Nos.5,254,840 and 5,194,719. Alternatively, heaters comprised of sheet metaland fabricated by wrapping metal foil into channeled honeycombconfigurations can be used.

In the case of extruded metal honeycomb heating elements, such as bestillustrated in FIGS. 1 and 2 of the drawing, the particularly preferredconfiguration is a round disk 10 of extruded metal honeycomb materialhaving the channels or cells running axially through the thickness ofthe disk. Slots exemplified by slots 12 are cut into the edges of thedisk through the disk cross-section by removing some of the cell walls.These slots are designed to create a serpentine conductive path acrossthe diameter of the disk between powering electrodes, in order toincrease the electrical path length and resistance of the disk for moreefficient electrical heating at typical motor vehicle battery oralternator voltages.

Powering electrodes or leads in the form of heavy metal leads or studs14 are welded to opposing edges of the metal honeycomb disk 10 atopposite ends of the serpentine path. These leads serve as theelectrodes for connection to an electrical power source. In theembodiment shown, each lead is encased in insulating ceramic material14a to electrically isolate it from close-fitting flared tube electrodesealing elements 15 attached to enclosure 16 in which the heater iscontained.

As is known, slots 12 in honeycomb 10 are preferably kept open by theinsertion of insulating separators. Conventionally, these are formed ofa refractory electrically insulating material such as an aluminaceramic, and are retained in holes drilled into each slot from theperimeter of the honeycomb. As hereinafter more fully described,insulating separators formed of dielectrically coated metal tabs providea more vibration-resistant heating element for the present purpose.

To electrically insulate and mechanically isolate the metal honeycombfrom surrounding enclosure 16, a layer of resilient insulating mountingmaterial 20 is provided around the honeycomb. This layer, which may beformed of any electrically insulating, refractory, woven or non-wovenresilient material, must be sufficiently refractory to resistdeterioration at maximum exhaust system temperatures and sufficientlydurable to withstand prolonged vibration and moderate to severemechanical shocks.

The mounting material must also be sufficiently resilient to transmitthe required level of diametrically balanced preloading force to thehoneycomb heater, and to retain that force at high use temperatures.Preferred materials for the resilient mounting material are woven matmaterials formed of refractory fibers, although non-woven mats or evenresilient insulating foam materials, if sufficiently resilient andrefractory, could alternatively be employed.

In the preferred embodiment of FIGS. 1 and 2, tubular metal enclosure 16is formed as a one-piece cylindrical steel tube of circularcross-section surrounding heater 10, with the heater being disposedwithin and across the cylindrical channel formed by the tube. Alsopositioned within tube 16 is a circumferential support member 22 forheater 10, that member being permanently connected to an inner surfaceof the wall of enclosure 16.

Connection of the support member 22 to the wall of enclosure 16 can beby means of welding or other permanent bonding. Alternatively, thesupport can be directly formed in the wall by shaping the wall toprovide the necessary peripheral support surface for the heater. Ineither case, the heater support surface extends sufficiently inwardlyinto the channel formed by the cylindrical wall of tube 16 so that itcan provide the necessary edge support for the peripheral edge of theheating element.

Opposing the support member within the channel, i.e., on the side of theheater opposite the support member, is peripheral compression ring 24.As shown in FIG. 2, compression ring 24 will extend from the wall oftube 16 into the channel a distance sufficient to provide a secondperipheral support surface of substantially the same size and shape asthat provided by support member 22 which it opposes. Compression ring 24is attached to the wall of tube 16 at a position such that, incooperation with support member 22 and a resilient mounting material 20,it applies a diametrically balanced compressive force on peripheral edgeportions of heater 10 which are positioned between support 22 andcompression ring 24. It is important to provide one or more layers ofresilient mounting material 20 between those edge portions and each ofsupport member 22, compression ring 24, and the inner surface of tube16, in order to develop the necessary compressive force on heater 10without damaging the peripheral edge thereof.

The spacing of compression ring 24 from support member 22 is set todevelop a predetermined level of compressive force on heater 10. Thatforce is at a level sufficient to firmly contain the heater, and isbalanced to insure extended heater life. Typically, preloading forces inthe range of about 668-2670N (150-600 lbf), generating pressures in the620-2760 kPa (90-400 psi) range on the edge portions of the honeycombssupported by the circumferential support member and compression ring,are used. These forces will provide the shock protection necessary forextended heater life provided that the forces are diametrically balancedacross the heater in the final assembly, as hereinafter more fullydescribed.

Mounted heaters such as shown in FIGS. 1-2 are particularly useful topreheat honeycomb-supported catalysts mounted in close proximitythereto. Such catalysts will preferably be mounted in the sameenclosure, in the manner of catalyzed honeycomb 25 which is partiallyshown in cross-section in FIG. 2. The diameter of enclosure 16 can beadjusted upwardly or downwardly as needed to tightly contain honeycomb25 within a resilient mounting material 26 closely behind heater element10, to obtain the most efficient heating of the catalyst.

A method and apparatus for mounting an electrical heater in a one-piececylindrical enclosure as above described are illustrated in FIGS. 3-5 ofthe drawings. The apparatus comprises tooling for developing a balancedor uniform preloading force across the heater as the compression ring isattached to the enclosure.

As shown in FIGS. 3 and 4, a metal honeycomb heater element 10 providedwith an edge covering of a refractory resilient mat material 20 is setinto cylindrical metal enclosure 16 and across the opening in thatenclosure. The heater and mat are positioned so that the mat and edgeportion of heater 10 are resting on the peripheral support surfaceprovided by the extension of support member 22 into the openingcontaining the heater. Mat 20 also separates heater 10 from the adjacentwall portion of enclosure 16.

Compression ring 24 is placed in the enclosure so that it covers theperipheral edge of heater 10, but with mat 20 positioned between thering and heater. Compression tooling consisting of a pressure plate 30and post 32 is then placed into the opening and over compression ring 24so that the plate is centered on the ring and contacts it uniformlyabout its periphery.

A compressive force in the range of about 668-2670N (150-600 lbf) isapplied to the post and pressure plate to urge compression ring 24against the insulation material. This force is sufficient to generate acompressive force in the range of about 620-2760 kPa (90-400 psi) onopposing sides of heater 10. Due to the rigidity of the tooling and thecentering of the pressure plate on the ring, the compressive forceapplied to the heater is diametrically balanced across the heater. Thatis, the compressive force at any one point on the heater periphery is ofsubstantially the same magnitude as the compressive force at thediametrically opposite point.

To maintain the level of balanced force on heater element 10, throughthe compression ring attachment procedure and during heater use,compression ring 24 is permanently affixed to the inner wall ofenclosure 16 by multiple-point attachment, rather than seam attachment,of ring 24 to the wall. As best seen in FIG. 4, to facilitate multiplepoint attachment, pressure plate 30 is provided with multiple radiallyoutwardly extending protrusions such as protrusions 34, these beingseparated by radially inwardly extending recesses or cutouts located atselected locations about the plate circumference.

The recesses extend inwardly from the outer circumference of thepressure plate defined by protrusions 34, and thus expose the joint 36between the wall of enclosure 16 and ring 24 to facilitate attachment ofthe ring to the wall. For example, exposure of the joint offers room forwelding or other fastening at selected points while still maintainingthe initially applied controlled heater preloading force on the ring.

The attachment of the ring to the wall is most preferably accomplishedby TIG (tungsten inert gas) welding, although any other attachmentmethod which avoids excessive compaction or shrinkage of the compressionring, ring-wall attachment points, and/or bonding material couldequivalently be used. Welds 38 in FIG. 4 are illustrative of onemultiple-point attachment pattern, although other patterns, includingwelding at all of the recesses about the periphery of the ring, could beused. In this regard it is important to avoid seam welding approaches tomaintain compression, as these approaches typically involve substantialweld compaction effects which are thought to significantly interferewith achieving and maintaining the required levels of compression on theheater.

FIG. 5 of the drawing illustrates an alternative form of a pressureplate, in this case a plate 30a of generally square configuration. Thisconfiguration forms four cutouts for ring/wall access separated by fourradially outwardly extending protrusions (corners) 34a which applypredetermined compressive force to ring 24 and heater 10. When urgedagainst the ring, this plate applies balanced but preferential force tofour opposing sections of the ring and heater element.

It is also important that preferential loading as provided by thepressure plate of FIG. 5 be applied at least to points on the heaterperiphery where the honeycomb separator slots and associated beams ofthe metal honeycomb heater are at their longest, i.e., spanning thegreatest distance across the channel of enclosure 16. These sections ofthe heater are thought to exhibit the greatest susceptibility to flexureand damage during the hot vibration encountered in use, and somaintaining proper edge compression at these locations is particularlycritical to extended heater life.

As noted above, the preferred conductive metal honeycomb heating elementused in these assemblies is an extruded metal honeycomb in the form of aflat disk configured as disk 10 in FIGS. 1 and 2, this disk being ofpredetermined diameter and thickness and comprising multiplethrough-channels traversing the thickness. As shown in FIGS. 1 and 6,the honeycomb disk incorporates multiple slots 12 extending inwardlyfrom the diameter, these conventionally being held separate along theirlength by dielectric rod or pin separators located near the disk outerdiameter, such as shown in U.S. Pat. No. 5,194,719.

It has now been found that, under severe vibration, such dielectric pinscan loosen and shift, inflicting damage on the metal honeycomb. Forimproved protection from vibration damage, then, heater units providedaccording to the invention incorporate anchored tab slot separatorsinstead of the dielectric pins of the aforementioned patent. Theseseparators are most preferably formed of sheet metal and are providedwith a dielectric coating at least on portions thereof in contact withwall segments of the slots. The segments of the separators extendingfrom the slots along the circumferential surface of the heating elementprovide the means for permanently attaching or anchoring the separatorsto the heater surface, to prevent shifting in use.

FIG. 6 illustrates two different configurations for such anchored slotseparators. Separators 17 are angled flat components including foldedflat insulated segments extending downwardly into slots 12 and flatmetal segments extending from the slots along the circumferential outersurface of heater 10, the latter segments being permanently attached tothe heater, e.g., by welding. As shown in more detail in FIG. 6a,separator 19 is similarly fastened to the surface of the heatingelement, but includes a cylindrical rather than flat insulated endextending downwardly into slot 12.

The configuration of separator 19 offers a particular advantage in thatthe segments for fitting into the slots are cylindrical. As aconsequence, these slot separators will fit into any slot regardless ofthe angle between the slot and the perimeter surface of the heater.Separator 17, in contrast, must be specifically shaped for each slotlocation, in order that the angle between the slot-fitting segment andextending portion is set to obtain the best fit to both the slot and theheater perimeter at that location.

A suitable dielectric coating material for the downwardly extending endsof these separators is alumina, although other dielectric coatingmaterials exhibiting adequate temperature and durability characteristicscould be used. Examples of suitable metals for the tabs includeKanthal(TM) metal foil and stainless steel.

As noted above, heating elements 10 such as shown in FIGS. 1, 2 and 6are generally provided with one or more stud electrodes 14 extendingfrom the heater outwardly of the heater enclosure 16 for applyingelectrical power thereto. As best seen in FIG. 6, each metal studelectrode 14 is welded to the outer circumferential surface of theheater disk 10 and extends outwardly of the enclosure via a feedthroughin the wall of enclosure 16.

In a preferred embodiment of the heaters of the present invention, eachelectrode feedthrough includes a gas-tight assembly formed by theelectrode 14, the enclosure wall 16, and a flared metal tube fitting 15having a flared end 15a and a tubular end 15b. Tubular end 15b of thisfitting forms a gas-tight seal around dielectric coating 14a onelectrode 14, while flared end 15a is used to form a gas-tight sealagainst the wall of enclosure 16.

The seal between flared end 15a and wall member 16 may suitably beprovided by welding the end to the wall. An advantage of this type offitting is that the edge of flared end 15a is sufficiently spaced fromthe feedthrough opening that damage to internal heater components duringwelding is minimized.

The seal between tube end 15b and electrode 14 with coating 14a can beobtained by the introduction of sealing material between the coating andtube, or by a shrink-fitting procedure. The latter procedure comprisesheating the fitting to expand it, e.g., to 800° C., and then dropping itover the electrode and onto the enclosure wall where it contracts oncooling to form a gas-tight seal against electrode coating 14a.

The invention will be further described by the following example, whichis intended to be illustrative rather than limiting.

EXAMPLE

An extruded metal honeycomb disc for a heater element, being about 9.3cm (3.66 in) in diameter and 0.762 cm (0.300 in) in thickness andincorporating edge slotting as seen in FIG. 1 of the drawing isprovided. To opposing outer edges of this honeycomb disc are attachedtwo opposing electrodes for electrical contact with the disc, eachelectrode consisting of a stainless steel stud about 8 mm in diameterwhich is welded to the disc for electrical contact. Each stud supportsan insulating ceramic coating on its side surfaces.

A wrap of woven insulation in the form of a fibrous mat is draped aroundthe perimeter of the honeycomb. This mat, formed of Nextel® ceramicfiber mat material commercially available from the 3M Company,Minneapolis, Minn., has insulating and electrical characteristicssuitable for thermally and electrically isolating the heater fromsurrounding metal. The wrap includes opposing holes through which theelectrode studs can protrude.

The wrapped heater element thus provided is inserted with itssurrounding wrap into the end opening of a cylindrical stainless steeltube of circular cross-section about 9.9 cm (3.9 in) in diameter such asshown in FIGS. 1 and 2 of the drawing. Within the channel formed by thetube is a circular support member in the form of a flat stainless steelring about 0.318 cm (0.125 in) in thickness, 9.88 cm (3.89 in) in outerdiameter, and 8.2 cm (3.24 in) in inner diameter, welded to the insideof the tube wall. This ring is disposed with its center axis parallelwith the center axis of the channel. The tube sidewall is provided withslots to accommodate the electrodes, these slots then being subsequentlyfilled by welding slot covers into the open slots.

The wrapped heater element is positioned on the ring support member anda second or compression ring of dimensions similar to that of the firstring is positioned over the wrapped heater element. A tool consisting ofa compression plate and post is then placed over the compression ringand in edge contact with the ring but not the heater. The compressionplate has 12 radially inwardly extending recesses about itscircumference which expose the width of the compression ring and the gapbetween the ring and the wall of the steel tube.

A force of 1335 N (300 lbf) is applied to the post and pressure plate toforce the compression ring toward the heater and support ring, thisforce being sufficient to develop a pressure of about 896 kPa (130 psi)on the edge surfaces of the metal heater element covered by the rings.While maintaining this pressure the compression ring is welded to theinner wall of the tube at each of the twelve points about thecircumference of the ring located opposite the recesses on thecompression plate, using TIG welding apparatus. The TIG welds aresufficiently small that no significant movement of the ring due to weldcompaction occurs. Therefore, the balanced compressive force developedby the pressure plate on the heater periphery is maintained.

Durability testing of the heater contained within the enclosure as abovedescribed is carried out under environmental conditions designed toapproximate those encountered in an automotive exhaust environment. Thetests used are vibration tests, involving hot (900° C.) and ambienttemperature vibration of assembled units at vibration rates of 100 and185 hertz and at accelerations of 28 and 60 G. For the hot vibrationtests, gas at the temperature noted is passed through the assembledheater enclosure until an equilibrium temperature distribution isreached. The axis of vibration of the units is either axial (parallelwith the flow-through axis of the honeycomb heater) or at a 45° angularoffset from the flow-through axis. Test durations range from 10-100hours.

Vibration testing applies repeated flexural stress to the heaterelement, and can be particularly damaging at the higher test temperaturewhere metals are more susceptible to damage from fatigue. The 45°vibration test is significantly more severe than the axial vibrationtest because of the shear forces exerted on the honeycomb heater and matby the compression rings during vibration. A mounted honeycomb heaterunit is considered to pass a vibration test if it exhibits nosignificant honeycomb damage after the test, as determined from bothvisual examination and electrical testing of the heater.

Enclosed heater elements produced in accordance with the foregoingExample, i.e., mounted using compression tooling such as disclosed inFIG. 4 of the drawing, exhibit significantly improved resistance to 45°vibration damage. Typically, such units can readily pass vibrationtesting which includes 10 hours of hot (900° C.) vibration at 45°, 60 gand 185 Hz, followed by 30 hours of the same vibration at roomtemperature.

Still further enhancements in heater durability can be realized usingcompression plate mounting as illustrated in FIG. 5 of the drawing. Thefollowing Table sets forth the results of tests conducted on heaterunits provided using such mounting, the mounting force of approximately670 N (150 lbf) having been preferentially applied to the compressionrings at points proximate to the slot or beam ends of the slottedhoneycomb heaters to be mounted, and at points 90° offset therefrom.While maintaining this force, attachment of the compression ring to theenclosure wall by TIG welding at eight points proximate to the extendingcorners of the compression plate, as illustrated in FIG. 5 of thedrawing, completed the mounting of the heaters.

Included in the Table for each of the units tested are the accelerationforce applied during vibration, the vibration frequency, the temperatureof the unit during testing, the vibration axis for the test, whetheraxial (parallel with the enclosure axis of the unit) or at a 45° anglefrom the enclosure axis, and whether the unit passed (P) or failed (F)the combined electrical/mechanical examination at the end of the test.As is evident from the Table, very high (100 hour) resistance to 45° hotvibration damage can be achieved by the application of balancedpreferential mounting pressure at the beam ends of the heater as abovedescribed.

                  TABLE I    ______________________________________    Vibration Tests - Compression Plate Mounting    Accel- Vibration Temp.          Vibration                                           Pass (P)/    eration           Frequency (°C.)                             Duration                                    Direction                                           Fail (F)    ______________________________________    28 g   100Hz     900° C.                             100 hr axial  P    28 g   100HZ     900° C.                             100 hr 45°                                           P    60 g   185Hz     900° C.                              10 hr axial  P    60 g   185Hz     900° C.                             100 hr 45°                                           P    60 g   185Hz     RT       30 hr 45°                                           P    ______________________________________

Prior art heater mounting methods which do not use balanced compressionapproaches tend to produce mounted units resistant to axial vibrationdamage, but overly susceptible to damage under off-axis vibration. TableII below sets forth vibration test results for enclosed heatersassembled by axially compressing the honeycomb heater elements betweenenclosure halves containing fixed stops for heater support. We attributethe failures in 45° vibration testing of such units to the difficulty ofdeveloping and maintaining balanced mounting pressure on the heaterelements during assembly of the enclosures.

                  TABLE II    ______________________________________    Vibration Tests - Comparative Samples    Accel- Vibration Temp.          Vibration                                           Pass (P)/    eration           Frequency (°C.)                             Duration                                    Direction                                           Fail (F)    ______________________________________    28 g   100Hz     900° C.                             100 hr axial  P    28 g   100Hz     900° C.                             100 hr 45°                                           F    60 g   185Hz     900° C.                             10 hr  axial  P    60 g   185Hz     900° C.                             10 hr  45°                                           F    60 g   185Hz     RT      30 hr  45°                                           F    ______________________________________

The probability of vibration failure appears to increase in the case oflow heater compression adjacent the beam or slot ends of slottedheaters. For example, one early procedure, comprising the application ofmounting pressure at points 45° offset from the beam ends of the heater,produced enclosed units failing within 30 minutes under hot (950° C.)vibration at 185 Hz (45°) and 60G. Even at ambient temperatures, heaterfailure occurred within 5 hours under these vibration conditions.

We claim:
 1. An electric heater module for heating a gas stream whichcomprises:a cylindrical metal enclosure comprising a wall member ofone-piece, closed-curved configuration forming a channel through theenclosure; a circumferential support member connected to an innersurface of the wall which extends into the channel to form a firstperipheral support surface; a conductive metal honeycomb heating elementpositioned across the channel adjacent the support member and having afirst peripheral edge surface in proximity to the inner surface andfirst peripheral support surface; a peripheral compression ring disposedin the channel in proximity to the heating element, the ring beingattached to the inner surface of the wall and extending into the channelto form a second peripheral support surface facing a second peripheraledge surface of the heating element; at least one layer of refractoryresilient insulation material disposed between the edge surfaces of theheating element and each of the inner surface of the wall and the firstand second peripheral support surfaces; wherein the compression ring ispositioned against and fastened to the inner wall by point attachmentsat a location maintaining a balanced axial compressive force on theinsulation material and peripheral edge surfaces of the heating element,said compressive force being diametrically balanced across the diameterof the heating element.
 2. An electric heater module in accordance withclaim 1 wherein the compression ring is positioned to apply a balancedpressure in the range of about 620-2760 kPa (90-400 psi) to the heatingelement.
 3. An electric heater module in accordance with claim 1 whereinthe conductive metal honeycomb heating element comprises an extrudedmetal honeycomb of flat disk configuration and predetermined diameterand thickness, the disk comprising multiple through-channels traversingthe thickness.
 4. An electric heater module in accordance with claim 3which comprises at least one stud electrode attached to an outercircumferential surface of the disk and extending outwardly of theenclosure through a gas-tight feedthrough in the wall member.
 5. Anelectric heater module in accordance with claim 4 wherein the gas-tightfeedthrough comprises a flared metal tube fitting having a flared endand a tubular end, the tubular end forming a gas-tight seal around theelectrode and the flared end forming a gas-tight seal against the wallmember.
 6. An electric heater module in accordance with claim 1 whereinthe extruded metal honeycomb incorporates multiple slots extendinginwardly from the diameter, the slots being separated adjacent thediameter by tab slot separators.
 7. An electric heater module inaccordance with claim 5 wherein the tab slot separators are formed ofmetal and are at least partially coated with a refractory dielectric.